Reliable navigation systems have always been essential for estimating position during flight. For example, traditional altimeter measurements determine the altitude of an aircraft above a fixed level by measuring air pressure (where air pressure decreases with an increase of altitude). A radar altimeter measures altitude by using the time for a radio signal to reflect from a surface (terrain) back to the aircraft. For example, the radar altimeter measures exact height during landing and when the aircraft (in particular, a rotary-wing aircraft) is in a “hover” mode. Radar altimeters are typically included as a component in various avionics and positioning systems. In each system, the radar altimeter component informs an operator (pilot) that the aircraft is flying too low or that terrain is rising to meet the aircraft.
Traditional radar altimeters use a closed-loop gain control to improve altitude accuracy. Any reductions in radar signal strength (that is, attenuation of the radar signal) limits the gain control and directly impacts altitude measurement accuracy. Several factors are typically analyzed to correct this reduction in signal strength, including ambient temperature, terrain conditions and aircraft altitudes. Maintaining proper signal strength levels is essential for accurate and reliable altitude measurement processing.